1. Field of the Invention
The present invention relates to the field of solenoid actuated spool valves.
2. Prior Art
Solenoid actuated spool valves are well known in the prior art. Such valves include single solenoid spring return valves and double solenoid valves, either of which may or may not incorporate magnetic latching. An example of such valves may be found in U.S. Pat. No. 5,640,987. Also known are two-stage spool valve systems, the first stage being a solenoid valve that hydraulically controls a second stage spool valve. See for instance U.S. Pat. No. 6,739,293.
In certain applications, such as in fuel injectors and hydraulic valve actuation systems, solenoid actuated spool valves must have a useful life of billions of cycles. This requires that the wear of the various parts be held to a minimum, in turn requiring hardened steels, such as by way of example, 52100 or 440C. These materials, however, have a relatively low magnetic field saturation density in comparison to the saturation density of physically soft magnetic steels (iron), such as annealed 1020. These steels may have a saturation density of as much as twice the saturation density of the hardened steels, and since magnetic forces are proportional to the square of the flux density, may provide approximately four times the maximum actuation force provided by the hardened steels for the same pole area.